The normal human cornea of the eye has a generally spherical configuration, and when contact lenses are fitted in such eyes, it is standard procedure to use a lens which has a generally spherical base curve. A complication is introduced when the eye being fitted has corneal toricity, and the toricity is great enough to prevent the use of a standard spherical base curve in the lens.
One of the methods frequently used for overcoming this complication is to provide a lens having a toric curve generally matching the configuration of the toric corneal surface. Another method is to provide a lens having a toric surface as an optical correction on the front surface of the lens. The use of such lenses of course adds another prerequisite to the configuration of the lens--namely, that it be essentially non-rotating in the eye, so that the toric axis in the lens remains aligned with the toric axis of the cornea. A commonly used technique for maintaining this meridional orientation of the lens is to provide the lens with ballast, so that gravity causes the lens to seek a natural position adjacent the bottom eyelid. The use of ballast in turn dictates that the lens be manufactured with a predetermined relationship or offset between the cylinder axis of the toric surface and the orientation of the ballast.
The current commercial procedure for manufacturing lenses of the above nature is heavily dependent on the skill and reliability of the lathe operator, who must make numerous precision measurements and adjustments in the preparation of each individual lens. Thus, for example, in the current procedure for base curve torics, the raw lens button is first sized and then mounted in a lathe collet where a sphere base curve is cut. Following this, the button is crimped (bent in a vise) until the desired toric cylinder is attained, as viewed in a radiuscope, and then the crimped button is ground to a sphere and de-crimped to create the toric surface. Next, the operator aligns the cylinder axis with a radiuscope and marks the button to the desired axis, and finally the button is re-mounted in the lathe collet, where the spherical cut for power and the cut for ballast are performed on the front surface. Mounting and blocking of the button, and use of the radiuscope for creating and marking the toric axis, are steps which demand precision on the part of the operators, and the cumulative effect of variations in each of the steps can easily lead to off-target axes and rejection of the lenses. Further, the stress created on the button during the crimping and decrimping steps results in unavoidable creep which is demonstrated by almost all glassy polymers under stress; hence additional power and cylinder variations are introduced.
When it is considered that lens making companies routinely keep stocks of contact lenses numbering in the tens of thousands, and the lathe operators have a full time job of maintaining such inventories by constant reiteration of the above-described procedure throughout the day for each type of lens produced, it will be understood that eye-strain and fatigue take their toll on the precision and skill of the operators. As a result of all the above factors, current production procedures can yield no more than 5% to 15% on target results for power, axis offset and cylinder.
The prior art includes various approaches for reducing this undesirably high scrap rate. For example, Neefe U.S. Pat. Nos. 4,188,353, 4,229,390 and 4,239,712 describe methods and apparatus for molding, rather than lathing, the base curve of contact lenses. The procedure involves casting a liquid monomer in a plastic cup-like molding cavity having the desired base curve in the bottom, polymerizing the monomer to form a lens button with the base curve thus formed, and then using the mold, with the lens button still retained, as a lathe arbor to machine the desired convex curves on the anterior surface of the lens. The procedure partially reduces the work-load of the lathe operator, since the precision steps of sizing, crimping and grinding to sphere are accomplished in the molding step rather than in the lathing. However, if the procedure is applied to the production of lenses with toric base curves, no means are provided for locating the toric axis or orienting it with respect to the axis of the ballast. The operator must still, with respect to each individual lens, use the radiuscope to locate and mark the toric axis, and then make the necessary lathe adjustments for each lens before performing the desired power and ballast cuts.
Kunzler U.S. Pat. No. 4,749,530 suggests an improvement over the above, involving modification of the cup-like molding cavity to include a hollow hemispherical base having one or more key ways or slots on the exterior surface. After liquid monomer is polymerized in the cup-like cavity to produce a lens blank having the desired base curve, the mold containing the lens blank is placed on a lens generating lathe having a hemispherical shaped locating ring, in such manner that the key way on the exterior surface of the mold base mates with a guide key on the locating ring of the lathe. In this manner, the location of the posterior lens surface curves is established when the mold is mounted on the lathe. Thus, the hemispherical-shaped locating ring of the lathe can be set at one position, and the front surfaces of successive similar lenses can be cut without remeasuring the base curve axis and resetting the cutting tool for each lens. However, this approach still requires lathe operator intervention when switching between lenses having different toric axes, since the hemispherical-shaped locating ring of the lathe must be reset for each axis. Although this may be useful when limited numbers of different lenses are being produced, it presents an inefficiency and an opportunity for operator error in large volume production or when automated production lines are desired. In addition, Kunzler's design is limited to lenses requiring small amounts of prism ballast. Thus, an operation involving double slab off ballast cannot be done by Kunzler.
It is an object of the present invention to provide a method for producing toric base curve lens casting molds, which method allows all toric base curve cutting, measuring and alignment steps to be transferred from the lathe operator to a preliminary automatic injection molding procedure in which no operator dependent determinations are required.
It is a further object of the invention to provide for the production of toric base curve lens casting molds which eliminate the need for lathe re-setting between batches of lenses having differing toric axes.
It is another object of the invention to provide for the production of toric base curve lens casting molds having lathe alignment means permitting an automated lathing operation in which successive batches of lenses having differing ballast to toric axis orientation may be machined on their front surfaces without making any readjustments on the lathe.
It is a still further object to provide injection molding apparatus for accomplishing the above.
Other objects and advantages will be apparent from the following description and drawings.